Field
The disclosure relates generally to a bandgap voltage reference circuit and, more particularly, to a voltage reference circuit device with a flexible output setting, over a range of high voltage supply rails.
Description of the Related Art
Voltage reference circuits are a type of circuit used in conjunction with semiconductor devices, integrated circuits (IC), and other applications. Voltage reference circuits can be classified into different categories. A category of voltage reference circuits are known as bandgap reference circuits. The input supply voltage levels change widely depending on the application in portable devices. For example, the supply voltage can be as high as 26V for notebooks, whereas in netbooks or tablets, the supply voltage is around 12V and in handheld devices it is generally 5V. Whatever the supply voltage level is, there is always a need for a fixed reference voltage. This reference voltage is generally very accurate (e.g. the bandgap voltage) and used all over the circuit where accurate reference needed regardless of the supply levels.
Power management circuits in particular are special cases since they also deliver the supply voltages and currents to the rest of the circuits in portable devices. During their operation, after supply voltages settle down, power management circuits also use reference voltage levels for various purposes similar to other type of circuits. However, during startup, since there is no regulated supply voltage available, a special type of circuit which generates the reference voltage has to be used. These blocks generally addressed as “crude bandgap” circuit blocks. As the name of the circuit implies, the goal is to provide a crude reference voltage during startup phase since accurate levels are not needed during that stage of operation. In summary, output of this reference circuit needs to be just accurate enough to start the circuit properly but at the same time it must prevent any breakdown voltage limitation for the transistors.
The current practice is to generate the proportional to absolute temperature (PTAT) current across a resistor with differential in the base-emitter voltage (ΔVBE) of two bipolar junction transistors (BJTs) with different emitter areas. For the PTAT generation, ΔVBE of two BJTs with an emitter area ratio of A is
      Δ    ⁢                  ⁢          V      BE        =            kT      q        ⁢                  ln        ⁡                  (          A          )                    .      
As a result, the same current through another resistor and also a diode connected BJT generates a reference voltage, which is equal to the bandgap voltage of the silicon. For this purpose, the complementary to absolute temperature (CTAT) dependence of a base-emitter voltage to temperature is used as
            V      BE        ⁡          (              T        ,                  I          C                    )        =            kT      q        ⁢                  log        ⁡                  (                                    I              C                                                      I                S                            ⁡                              (                T                )                                              )                    .      
In practical integrated circuits, VBE changes inversely proportional to temperature at roughly −2.2 mV/C, and KT/q is PTAT that has a temperature coefficient around +0.085 mV/C.
FIG. 1 illustrates a topology known to the inventors of a bandgap generator circuit 100 between voltage VDD 101 and ground VSS 102. The circuit 100 comprises a startup block 105 coupled to npn bipolar junction transistor (BJT) current mirror 120 with transistor Q1 125A of size A and transistor 125B of size xA. The current mirror 120 is coupled to resistor R1 127. The current mirror 120 is coupled to p-channel MOSFET current mirror M1 115A and M2 115B. The drain of M2 115B is coupled to the gate of p-channel MOSFET M2 130. Diode-connected BJT Q3 140 is coupled to resistor R2 145. The PTAT current is formed via R1 127 and is then copied over to R2 145. The combination of voltage over R2 145 and VBE of Q3 140 provides the reference voltage. Since VBE has a negative temperature coefficient and VR2 has a positive temperature coefficient the resulting effect is temperature independent. This reference voltage is equal to a silicon bandgap voltage.
The primary object of this methodology is to provide a reference voltage set to a fixed value equal to a silicon bandgap voltage. The drawback of this implementation is the silicon bandgap voltage is different from the desired reference voltages. In addition, the PTAT current across a diode-connected bipolar transistor is not a pure linear CTAT reference; there is a logarithmic temperature dependency which introduces circuit design challenges. The disadvantages of this implementation to achieve a voltage reference circuit includes a fixed non-adjustable bandgap reference and startup issues.
U.S. Patent Application 2014/002052 to Schaffer et al describes a circuit with an element with a negative temperature coefficient, and a second element with a positive temperature coefficient which are combined to produce a temperature coefficient. This application provides an inherently accurate adjustable switched capacitor voltage reference.
U.S. Pat. No. 8,547,165 to Bernardinis describes a method and system for a voltage reference produced from a PTAT, CTAT, and nonlinear current components generated in isolation of each other and combined to create the voltage reference. This is an adjustable second order compensation bandgap reference.
U.S. Pat. No. 8,278,994 to Kung et al shows a temperature independent reference circuit with a first and second bipolar transistor with commonly coupled bases with a first and second resistor.
U.S. Pat. No. 6,677,808 to Sean et al describes a voltage reference utilizing CMOS parasitic bipolar transistors where the transistors are coupled configured to generate a ΔVbe and Vbe/R, and a resistor divider, to provide an adjustable temperature compensated reference signal.
U.S. Pat. No. 6,563,371 to Buckley III describes a current bandgap voltage reference with a first current source to generate a positive temperature coefficient, PTC, and a second current source to generate a negative temperature coefficient, NTC, to produce a temperature invariant reference voltage.
In the previously published article, “A CMOS Bandgap Reference Circuit with Sub-1V Operation,” IEEE Journal of Solid-State Circuit, Volume SC-34, No. 34, May 1999, pp. 670-674, a voltage reference circuit is discussed that operates at a sub-1V voltage level.
In the previously published article “Curvature-compensated BiCMOS Bandgap with 1V Supply Voltage,” Solid-State Circuit, 2001, describes a 1V BiCMOS circuit.
In the previously published article “Reference Voltage Driver for Low-Voltage CMOS A/D Converter,” Proceedings of the ICECS 2000, Vol. 1, 2000, pp. 28-31 describes an analog-to-digital converter.
In these prior art embodiments, the solution to improve the operability of a low voltage bandgap reference circuit utilized various alternative solutions.
It is desirable to provide a solution to address the disadvantages of operation of a fixed voltage bandgap voltage reference circuit.